Earthquakes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 163

types of lakes can result from earthquake triggered landslides. These are tem-
porary lakes formed after damming the main trunk stream in a river valley;
these features are ephemeral as river floods soon remove them. The second
type is a permanent lake that forms on tributaries; these can remain intact
forthousands of years because the tributaries have comparatively lower dis-
charges and hence erosive power. Two factors, therefore, determine the longevity
of a landslide-generated dam; the size of the landslide and the size of the river
dammed.
The chronology of landslide-generated lakes is often determined using radio-
carbon dating. Three approaches can be used: radiocarbon dating of wood in
theslide debris, dating of standing trees drowned by the lake and dating of sub-
merged soil horizons cored below lake sediments. It is possible to estimate the
cause and size of a palaeoearthquake if a sufficient number of landslide-dammed
lakes can be investigated. A series of landslide-dammed lakes of a single age can
be used as an indication of the area shaken in one earthquake event. If care is
taken in identifying these landslides they can provide a conservative estimate of
palaeoseismicity that does not rely on the identification of an active fault. How-
ever, dating only one dammed lake is insufficient to give any conclusive answers.
To establish the cause and size of an earthquake it is necessary to identify and
date many lakes within the affected area (Adams, 1981).
Lake sediments
Earthquakes can cause sediments, often from landslides or slope move-
ments, to be deposited into existing lakes and fiords. Records of prehistoric
earthquake activity can be obtained through the study of these sedimentary
sequences. The sediments are often in the form of turbidites which occur when
sediments slump either subaerially or subaqueously. Turbidites often show a dis-
tinctive stratigraphy known as a Bouma sequence (although the Bouma sequence
is now often replaced by facies models that separate low-density from high-
density turbidite flows) and can be easily recognised from other sedimentary
sequences. Apart from turbidites, sediments in these water bodies can also reflect
changes, typically increases, in sediment influx. These changes can result from
earthquake-induced erosion of the basin sides as a result of seiching (where
wavesare generated) and liquefaction of sediments and soils on slopes, as well
as landslides. It is important to note that similar rapid changes in sedimenta-
tion can also occur due to climatic changes, volcanic activity and non-seismic
effects. As with other sedimentary palaeoseismic records, it is important to rule
out these other potential causes before an assumption is made that the sedi-
mentary evidence is due to prehistoric earthquake activity.